Bimetric gravity

Bimetric gravity or bigravity refers to a class of modified mathematical theories of gravity (or gravitation) in which two metric tensors are used instead of one. The second metric may be introduced at high energies, with the implication that the speed of light could be energy-dependent. If the two metrics are dynamical and interact then there will be two graviton modes, one massive and one massless, and thus bimetric theories are closely related to massive gravity.